Prior art U.S. Pat. Nos. 4,518,056 and 3,966,188 whose detailed description and drawings are specifically incorporated by reference into this specification.
Tracked vehicles such as snowmobiles have rear suspension systems generally consisting of front and rear suspension arms pivotally mounted on a shaft rotatably connected to the frame of the snowmobile and a slide frame comprising a pair of laterally spaced slide rails or longitudinal skids interconnected transversely on opposite sides of the machine. The slide rails are in sliding contact with an endless belt which provides ice and snow surface contact and friction drive for the snowmobile. In many current arrangements, there are four suspension arms: a front pair of arms located on opposite sides of the snowmobile and pivotally connected to the forward end of the slide rails, and a separate rear pair of arms. Each pair is connected to the slide rails or to a bracket capable of pivoting movement. A shackle or sliding block mechanism interconnects the rear suspension arm and the slide rails to permit relative linear movement.
This configuration allows the front and rear suspension arms to operate independent of one another which is thought advantageous in the prior art because of favorable weight transfer characteristics which enhance traction. This independence, however, has been found to result in rough and unsteady rides for the rider, particularly when the rear suspension of the track encounters an elevated mound of ice or snow or the upward side of a depression. This detracts from the enjoyment and the utility of the vehicle since there are many areas which, when traversed, will unduly subject the rider(s) to severe jolts and stress.
The independence between the front and rear suspension arms adversely affects the snowmobile in several ways. First, track tension is not adequately maintained when there is extreme deflection of either one of the front or rear suspension arms. Particularly, when there is an excess of 7 inches of suspension arm travel measured vertically between the suspension arm connection to the chassis and the suspension arm connection to the slide rails. In prior art the lower rear arm pivot has been allowed to move freely in a rearward direction relative to the slide rail. This unregulated movment allows the variation in track tension to be larger than desireable as a result of the independent movement of the front and rear arms. When the opposite occurs and the lower rear arm pivot is allowed to move unregulated to a forward position the increase in variations also occurs. These variations can reduce the comfort, control, track life and ultimately the safety of the rider.
When the track tension is reduced due to the independent rearward movement of the lower rear arm pivot relative to the slide rail the track can derail or be forced off of it's guide system which can instantly stop the driving movement of the track resulting in possible loss of control of the vehicle by the rider.
When track tension is increased due to the independent forward movement of the lower rear arm pivot relative to the slide rail, the tension or stretching subjected to the track can damage the internal constuction of the track and internal or external crosslinks if used on track.
Second, it requires the associated springs and shock absorbers to be sprung and dampened stiffly because each must individually support the high loads when impact occurs at either the front or rear extreme of the slide rails. That is, because each suspension arm acts independently, it must be engineered to withstand and control the full impact of the bump and weight of the snowmobile by itself. This results in a normal ride that is less comfortable due to stiffness.
Third, when the front suspension arm deflects as it contacts a bump, the independent rear suspension arm remains in its ride position or fully extended position. This results in an angle of incidence between the slide rails and the bump. Unless the, impact is then large enough to compress the rear suspension arm spring and shock absorber assembly, thereby flattening the angle of incidence, the slide rails will act as a ramp forcing the rear of the snowmobile upward. That is, with the slide rails angled in an upward incline due to the independent deflection of the front suspension arm but not the rear suspension arm, the snowmobile will hop over the bump, imparting a secondary jolt which increases in intensity with the speed of the snowmobile.
Some prior art suspensions have been made to reduce the independent movement between the front and rear suspension arms in an attempt to diminish the adverse effects described above. However, such attempts have resulted in the creation of additional problems. One of these additional problems relates to traction of the snowmobile upon acceleration caused by the improper transfer of weight by the suspension assembly. When a snowmobile is rapidly accelerated, a drive sprocket driven by the engine creates an abrupt pulling force in the upper surface of the track. This tension reacts through the rear track support wheels resulting in a forwardly directed force to the slide rails. Therefore, upon rapid acceleration, the slide rails are urged forwardly. Additionally in the prior art systems limited success has been incorporated to adequately control track tension when verticle suspension displacements have been increased to over 7" of travel.